Traditionally techniques for joining metal components have come as a result of adapting techniques initially developed for joining of ferrous materials.
Joining of metals has largely been connected with fusion welding, where both the base metal and any filler material is melted by an electric arc, electron beam or laser beam, allowing metal to metal bonding to be achieved in the trailing part of the weld pool during crystallisation. In fusion welding only a fraction of the energy supplied contributes to the melting and thereby to bonding. Most of the energy supplied leads to a local heating of the base metal and the formation of a so-called heat-affected zone (in the literature commonly referred to as HAZ) around the weld joint. This zone represents a problem, because the resulting microstructural changes lead to a permanent mechanical degradation of the parent material. The properties of the weld zone will thus become the limiting factor in engineering design and, in practice, determine the load-bearing capacity of the component. In addition, the excess energy (i.e. heat) supplied leads to high residual stresses in the weld region as well as to global deformations and distortions. These problems are greater in aluminum welding than in steel welding, since the possibilities of taking the necessary precautionary actions, e.g. by modifying the HAZ microstructure through adjustment of the base material chemical composition, is more difficult in the former case.
In general, the use of more effective welding processes like laser welding and electron beam welding provides a much narrower HAZ, which in this respect represents a significant improvement. These techniques, however, introduce other problems related to the hot cracking resistance and pore formation in the fusion zone. In addition, they suffer from the disadvantage of more costly and less versatile equipment. Furthermore, the tolerance requirements are much more severe due to the fact that a filler material is usually not added.
In the past several attempts have been made to develop alternative techniques for joining of light metals, of which friction welding or a variant known as friction stir welding (FSW) probably is the most promising one. In FSW the two plates to be joined together are pressed firmly against each other while a rotating tool is moved along the interface (edge) between them, removing the oxide layer that—at least for aluminium—always will be present on the surface. Even though considerable frictional heating occurs at the interface between the rotating tool and the parent aluminium plates, the energy supplied, and thereby the heat generated, is less than in fusion welding, so that the base material near the joint will not melt and reach a liquid state. Friction stir welding is thus an example of a solid state joining technique, which represents a significant improvement compared to fusion welding, as several of the common problems are thereby reduced, namely development of high residual stresses and hot cracks, pore formation and a low corrosion resistance. On the other hand, this novel technique is encumbered with several disadvantages, one being the requirement that the surfaces to be joined need to exactly match each other, as there is no possibility of using a filler material. Another disadvantage is that the components to be joined must be pressed against each other with a considerable force, which means that the method requires heavy and rigid equipment. Finally, even this type of friction welding gives rise to the formation of a wide HAZ, where the resulting microstructural changes lead to permanent softening of the precipitation strengthened material.
Among other methods of joining brazing, riveting and adhesive bonding should be mentioned. One or more of these methods may be convenient for some areas of application, but, in general, they provide a low safety against failure and are therefore not realistic alternatives to welding in load or weight carrying constructions.